This invention relates generally to apparatus for determining depth-related locations within a borehole and, more specifically to methods and apparatus for locating coupling joints in a string of subsurface tubing or casing while the apparatus is caused to traverse the borehole.
In the process of completing most oil or gas wells, a string of conduit or "casing" is placed into the earth borehole and cemented in place by pumping a slurry of cement down the annulus between the borehole wall and the casing. The casing is run into the borehole on standard length sections with the diameter being determined by the size at which the borehole is drilled. The sections of casing are joined into a string by a variety of threaded coupling apparatus referred to generally as "collars". The variety of collars available include external collars, a separate threaded piece into which one end of two sections of casing are threaded, and flush joint collars where the end of one section of casing threads internally into the end of another section of casing.
Once casing is cemented into place within the borehole the formations can be logged by passing a logging instrument through the casing annulus and/or the casing can be perforated at suspected production zones as determined by commonly used logging methods. When traversing the cased borehole with a logging or perforating instrument, it is desirable to have a quick, easy and reliable method to monitor the location of the device along the vertical extent of the borehole. This can be done by determining the relative position of the device in relation to the casing collars. Such a determination can be made by using a casing collar indicating instrument.
Casing collar indicator instruments used in the industry typically emit a magnetic field by means of a permanent magnet located in a centralized body member or an oscillator circuit coupled to an electrical coil. Changes in the magnetic field resulting from variations in the magnetic reluctance caused by variation in casing mass, due to collars, are detected and an electrical signal indicative thereof is transmitted to a signal recovery ciruit. Such devices have provided less than reliable results in the past. The primary problem experienced with these devices is a lack of sensitivity required to indicate collars where the magnetic reluctance caused by the mass of the joint is not radically disproportionate from that of the casing string. Thus, it has proven difficult to adequately establish casing collar locations within the borehole.
These and other disadvantages are overcome with the present invention by providing a new and improved casing collar indicator which can be operated reliably in combination with both logging instruments or perforating guns in a centralized or decentralized position within the casing string.